Certain embodiments of the present invention are directed to circuits. More particularly, some embodiments of the invention provide brown-out detectors. Merely by way of example, some embodiments of the invention have been applied to an electronic system. But it would be recognized that the invention has a much broader range of applicability.
Traditionally, an electronic system or component needs a stable power supply to ensure the electronic system or component functions properly. If the power supply of the electronic system or component is unstable, the electronic system or component may be damaged or malfunctioned.
For example, in an electronic system including a microprocessor or microcontroller unit (MCU), a brown-out detector of the electronic system is configured to monitor a supply voltage of a power supply of the microprocessor or microcontroller unit in real-time, and to output a reset signal to prompt the microprocessor or microcontroller unit to reset when the supply voltage falls below a threshold voltage. The reset process of the microprocessor or microcontroller unit is quite important because the semiconductor components of the microprocessor or microcontroller unit have minimum operating voltage limits. If the microprocessor or microcontroller unit fails to reset properly, the data stored at the internal buffer or memory of the microprocessor or microcontroller unit can be lost, causing an error in the program of the microprocessor or microcontroller unit.
With advancements in technology, the use of mobile devices has become widespread, and many mobile devices use batteries to supply power. A brown-out detector is generally needed for a mobile device, which uses a battery to supply power, for monitoring the battery voltage in real-time such that the mobile device is notified to take action when the battery voltage falls below a predetermined threshold voltage value. It is desirable for the brown-out detector to have minimal current consumption. However, a traditional brown-out detector often utilizes a bandgap reference voltage source as a reference voltage source that has very low temperature drift, preventing current consumption of the traditional brown-out detector from being minimized and thus from being applied to low-power products.
FIG. 1 is an exemplary circuit diagram showing a conventional brown-out detector using a bandgap reference voltage source. As shown in FIG. 1, the brown-out detector 10 includes a resistor-based voltage divider 110, a bandgap reference voltage source 120, a voltage comparator 130, and a delay circuit 140. The voltage divider 110 generates a characteristic voltage Vres of a supply voltage VD, the bandgap reference voltage source 120 generates a reference voltage Vref that is constant and independent from the supply voltage VD and temperature variations. The voltage comparator 130 compares the characteristic voltage Vres and the reference voltage Vref. If Vres<Vref, the voltage comparator 130 outputs a logic high level to indicate that the supply voltage VD is smaller than a detection threshold voltage VDD.
As depicted by the circuit diagram shown in FIG. 1, the relationship between the detection threshold voltage VDD and the reference voltage Vref is as follows:
                                          V            DD                    ·                                    R              B                                                      R                A                            +                              R                B                                                    =                  V          ref                                    (                  Equation          ⁢                                          ⁢          1                )                                          V          DD                =                              V            ref                    ·                                                    R                A                            +                              R                B                                                    R              B                                                          (                  Equation          ⁢                                          ⁢          2                )            